AFRL extends serviceability of hydraulic lines

The US Air Force Research Laboratory (AFRL) recently completed an effort to help develop, test and validate a cold spray coating process for the life extension of aircraft hydraulic system components.

Cold spray is a technique that allows metal particles to be accelerated onto a surface through high-pressure application. The force of the impact bonds the metal to the surface without the need for temperatures as high as those typically associated with other deposition processes. The process will potentially support the replacement B-1 aircraft hydraulic lines, which are prone to chafing damage.

B-1 hydraulic lines are made of titanium, a strong, lightweight metal that can be bent and routed around tight spaces without collapsing upon itself. These qualities make it ideal for aircraft hydraulic systems. However, titanium does have drawbacks. It is a very surface-sensitive material and any nick or scratch can be detrimental to its overall material properties. To reduce the problems, the rapid innovation fund team began looking at the use of the cold spray process to apply a protective titanium layer to chafe-prone tubing areas. The thought behind this effort was that the sacrificial titanium layer could endure considerable wear while preventing harm to the material beneath.

Jeff Calcaterra, Structural Materials Evaluation Team Lead, said: ‘Titanium is very sensitive to temperature changes in the range in which cold spray is applied. Applying high heat to titanium tubes in an oxygen-rich environment causes a brittle crust—called alpha case—to form on the outside.

‘We developed a process in which we use helium as a carrier gas. The helium displaces oxygen, so the brittle layer doesn’t have a chance to form on the tube.’